The data presented here are based on over 700 mice and employed over 13,000 injections. The results obtained provide good evidence that the influences of sex steroids on fighting motivation in aggressive mice are discrete from the somatic influences of these hormones. Further, it seems possible that some of the effects of naturally occurring endogenous androgens in the mouse are consequent upon local neural aromatization of these compounds to estrogenic metabolites, before they exert an influence on aggressive motivation. It seems possible that this is a physiological effect as well as a consequence of pharmacological influence, as the doses of steroid employed in this present study are quite low. However, this model of hormone action cannot be the entire story, as 5 alpha DHT is behaviorally effective even though its A ring cannot be aromatized. Aggressive mice seem useful behavioral models for these effects. These studies indicate some of the likely sources of variability in the literature, as well as providing a number of considerations for clinical behavior therapy. They also indicate some of the features of steroid hormones (such as an aromatized A ring; the presence of a hydroxyl group at position 19; the influence of alpha hydroxyl groups, etc.) which may change the behavioral and somatic influences of these compounds.